Dalton's Law of Partial Pressure

Dalton's law of partial pressure states that total pressure of the mixture of inert gases is equal to the sum of partial pressures of each gas present in the mixture.

1. Dalton’s law of partial pressure XI FDC CHEMISTRY CHAPTER 4 SIDRA JAVED

2. Dalton’s Law of Partial Pressure The total pressure in a container is the sum of the pressure each gas would exert if it were alone in the container. The total pressure is the sum of the partial pressures. P Total = P 1 + P 2 + P 3 + P 4 + P 5 ... 2 John Dalton

3. 3 Dalton’s Law

4. Example of Dalton’s Law Air is a mixture of non reacting gases. 78% N 2 , 20% O 2 , 0.93 % Ar , 0.03% CO 2 , traced of Ne, He, Kr, H 2 , water vapors and pollutant gases such as oxides of sulphur and nitrogen are found in air. The total pressure exerted by air is equal to the sum of partial pressure of each gas. 4

5. Relationship between partial pressure and no. of moles of gas Let n A , n B and n C be the no. of moles of gas A, B and C respectively Partial pressures of these gases are P A , P B and P C As we know that, PV= nRT For Gas A, ….( i ) For Gas B, ….(ii) And for Gas C, ….(iii) T otal pressure, ….(iv) where n t = n A + n B + n C   Dividing equation ( i ) by (iv) = Similarly, and   5

6. Relationship between partial pressure and no. of mole FRACTION OF GAS We know that: or But, , where X is called mole fraction. So, P i = X i P t or P i α X i   6

7. Applications of Dalton’s Law of partial pressure

8. Collection of gases over water When a gas is collected over water, provided that the gas is insoluble in water, gas becomes moist. The total pressure of moist gas will be equal to: P t = P gas + P water Or P gas = P t – P water T he pressure due to water vapors is called aqueous tension.

9. Respiration at higher altitudes Partial pressure of O 2 in outside air (159 g/cm 2 ) is higher than partial pressure of O 2 in lungs (116g/cm 2 ) However at higher altitude, air pressure decreases, hence respiration becomes difficult. Pilots may have un-comfortable breathing in a non-pressurized cabin where the partial pressure of oxygen is about 150 g/cm 2

10. Respiration by deep sea divers At the depth of sea, air pressure increases 5 times than normal pressure. Regular air cannot be used in a diver’s tank because partial pressure O 2 would be 795 g/cm 2 . Deep sea divers use a mixture of 96% He and 4% O 2 to breath. SCUBA (self-contained breathing apparatus) contains compressed air to breath. As the divers returns to surface, it becomes hazardous for him if not handled properly. As the divers comes up, the pressure of surrounding water drops. Consequently the compressed air in lungs expands. Surfacing is done very slowly so that the compressed air may escape out from the lungs without damage to them.

11. How deep sea divers respire on return from sea? Air contains 80% N 2 . if N 2 is present in divers tank, the solubility of N 2 in blood can increase with increase in pressure in very deep sea. If the diver returns quickly and relatively at low pressure at surface, he will face a life threatening condition called Bends. The diver must use different mixture of gases such as He and O 2 or spend many hours decompression chamber after a dive. The pressure in decompression chamber is slowly lowered over many hours. N 2 comes out of blood slowly and disposed off safely.

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